Glow discharge deposited hydrogenated and fluorinated amorphous semiconductor alloy films have gained commerical acceptance as the material from which to fabricate low cost and efficient photovoltaic, semiconductor and electronic devices. Since Ovshinsky, et al first reported the development of efficient photoresponsive devices from amorphous silicon alloys in 1978, a great deal of effort has been expended worldwide by research teams in analyzing the properties of amorphous semiconductor alloy, particularly said amorphous silicon alloys, so as to optimize the types and percentages of constituient elements introduced into the glow discharge plasma, as well as the fabrication parameters utilized in the production of same. Up to the date of the instant invention, diborane was the precursor process gas utilized in research as well as commercial applications in order to substitutionally p-dope the semiconductor alloy host matrix for forming the p-doped semiconductor alloy layer of various photoresponsive devices. However, the p-doped semiconductor alloy layer is generally considered to be the poorest layer of a p-i-n type photoresponsive device. The p-doped layer is considered to be the poorest because of the fact that more tail states exist adjacent the valence band than adjacent the conduction band of an amorphous silicon alloy material. Consequently, the Fermi level of the p-doped layer cannot be shifted as close to the valence band thereof as the Fermi level of the n-doped layer can be shifted to the conduction band.
Due to the fact that (1) very little boron is substitutionally introduced into the host matrix of the semiconductor alloy material (vis-a-vis, the amount of boron alloyed into said host matrix) and (2) a very large number of tail states exist adjacent the valence band of amorphous silicon, it has not heretofore been possible to move the Fermi level of said p-doped amorphous silicon alloy material closer than approximately 0.3 electron volts (eV) from the valence band. This is in contrast to the n-doped material in which the Fermi level can be moved almost all the way up to the conduction band. Obviously, the closer to the respective band edges the Fermi level can be moved and better the degree of substitional doping of the semiconductor alloy, (1) the better the electrical conductivity of that semiconductor alloy material becomes and (2) the higher the strength of the electric field induced across the intrinsic semiconductor alloy material by the doped semiconductor alloy materials becomes. Based upon the foregoing discussion, it should be evident that (1) the p-doped amorphous semiconductor alloy layer indeed represents a weak link in the p-i-n or p-n type photovoltaic devices, and (2) any improvement in the p-doped semiconductor alloy layer which would increase the amount of substitutional doping thereof without significantly increasing the density of defect states in the band gap thereof would represent a clear step forward in the art.
Recently, considerable effort has been expended to develop systems and processes for depositing amorphous semiconductor alloy materials which encompass relatively large areas, and which can be doped so as to form p-type and n-type semiconductor alloy layers for the production of thin film p-n type and p-i-n type photovoltaic devices substantially operatively equivalent to their crystalline counterparts. It is to be noted that the term "amorphous", as used herein, includes all materials or alloys which have no long range order, although they may have short or intermediate range order or even contain, at times, crystalline inclusions. Of course, the photoresponsive devices so fabricated from doped and undoped amorphous semiconductor alloy layers were limited in the photoconversion efficiency thereof by, inter alia, the poor quality p-doped semiconductor alloy material. It should be noted that the morphology and growth of subsequently deposited thin film layers are also adversely affected by the poor quality of the p-doped layer and the problems of undesirable morphology and growth discussed hereinafter. Consequently, the deposition of a poor p-doped layer causes morphologically poor intrinsic and n-doped semiconductor alloy layers to be grown.
It is now possible to prepare amorphous silicon alloys by glow discharge or vacuum deposition techniques, said alloys possessing "acceptable" concentrations of localized defect states in the energy gaps thereof. While the concentrations of defect states have been termed "acceptable", the densities thereof remain too high to produce photovoltaic cells having photoconversion efficiencies in the 15% and above range. In order to better appreciate the full impact of the present invention in providing a more substitutionally doped p-type semiconductor alloy, a brief historical description of the mechanisms involved in the deposition of amorphous semiconductor alloy materials is presented.
The most commonly employed amorphous semiconductors, i.e. amorphous silicon and germanium, are normally four-fold coordinated and normally include microvoids and dangling bonds which are believed to produce said high density of localized states in the band gaps thereof. In the glow discharge deposition of amorphous silicon films, a silicon containing process gas such as silane flows into a reaction chamber for decomposition by an R.F. plasma and deposition onto a substrate. It is the elements present in the process gases, the chemical combinations of those elements following disassociation by the plasma, as well as the manner in which those elements are bonded into the host matrix of the semiconductor material that determine the density of defect states in the band gap of the deposited semiconductor alloy film; also important to the quality of the semiconductor alloy film is whether the dopant element and species introduced into the plasma are incorporated substitutionally or alloyed with the semiconductor material.
More particularly, it is now known that the hydrogen from the silane precursor gas which combines at optimum temperature and pressure conditions with many of the dangling bonds of the silicon during the glow discharge deposition process so as to substantially decrease the density of the localized states in the energy gap toward the end of making the deposited amorphous semiconductor alloy material approximate more nearly the corresponding crystalline semiconductor alloy material. However, the incorporation of hydrogen not only has limitations based upon the fixed ratio of hydrogen to silicon in the silane gas, but, perhaps most importantly, various Si-H bonding configurations operate to introduce new antibonding states into the semiconductor alloy material which can deleteriously affect the electrical and optical properties of said material. Therefore, there are certain unacceptable material properties which arise when hydrogen is utilized to reduce the density of localized states in these materials, which properties are particularly harmful in terms of the effective p as well as n doping thereof. The resulting unacceptable density of states of the hydrogenated silane-deposited semiconductor alloy materials leads to a narrow depletion width, which in turn limits the efficiencies of solar cells and other devices, the operation of which depends on the drift length of charge carriers through the layers of semiconductor alloy material.
Thus it has been attempted to alter the amorphous silicon, deposited from an atmosphere of silane by prior art glow discharge deposition processes, by the addition of hydrogen from the silane precursor gas in an effort to make said silicon more closely resemble crystalline silicon. To that end, the silicon is doped in a manner similar to the manner in which crystalline silicon is doped. However, such glow discharge deposited amorphous silicon has characteristics which, in all important respects, are inferior to those of doped crystalline silicon and therefore cannot be used successfully in place of doped crystalline silicon.
While the amorphous semiconductor alloy materials have many bonding options, the bonding of the elements of the amorphous semiconductor alloy material into the solid amorphous matrix is primarily accomplished by covalent bonding, which bonding is responsible for the strong bonds which allow the amorphous material to substantially maintain its integrity and energy gap. As used herein, the normal structural bonding, which characterizes conventionally prepared amorphous materials, is the condition where each atom forms the optimal number of bonds, such as covalent bonds, primarily responsible for the cohesive energy of the amorphous solid. The energy gap of a semiconductor alloy is basically determined by the solid amorphous semiconductor alloy materials forming the amorphous host matrix and the structural configurations present in that matrix. In purely substitutional doping, a dopant atom (such as boron for p-doping) takes the place of a semiconductor atom (such as silicon) in the host matrix in such a manner that (1) the bonding remains covalent (tetrahedrally coordinated), (2) the silicon to silicon and silicon to boron bonds are not strained, and (3) the concentration of higher order boron hydrides is minimized.
It is the situation which arises when other, weaker, bonds (vis-a-vis, the existing silicon to silicon bonds) are formed upon the introduction of the p-dopant atoms that gives rise to a solid amorphous semiconductor alloy material which has a wide spectrum of localized states in the energy gap, including bonding and nonbonding states, which states are herein referred to as deviant or defect electronic configurations and which have an effect upon the Fermi level, the electrical conductivity and the electrical activation energy of the semiconductor alloy material. Such deviant electronic configurations can include substitutional impurities and vacancies, intersitials, dislocations, and so forth, which can occur principally in crystalline solids because of periodic restraints therein. In solid amorphous alloy materials, three-dimensional orbital relationships can occur which are generally prohibited in crystalline materials by reason of the periodic lattice constraints in the latter. Other deviant electronic configurations, particularly in the amorphous semiconductor alloy materials described in the instant application, can include microvoids and dangling bonds, dangling bond and nearest neighbor interactions, lone pairs, lone-pair/lone-pair interactions, lone pair and nearest neighbor interactions, valence alternation pairs, dative or coordinate bonds, charge compensation, polyvalency, lone-pair compensation, hybridization, three-center bonding, pi-bonding, and others, all of which operate toward pinning and affecting the Fermi level in the energy gap of the semiconductor alloy materials and control the electrical conductivity mechanism within said materials.
The localized states present in the energy gap, the degree of substitutional doping as well as the growth and morphology which occurs, and the concentration of boron polymers and oligomers are, inter alia, related to the structural configuration of the host matrix of the amorphous semiconductor alloy, to the nearest neighbor relationships of the elements in that matrix, to the aforementioned deviant electronic configurations, and to the electrically active centers in the amorphous semiconductor alloy matrix. The electrical activation energy E.sub.a for free carrier conduction is usually the energy difference between the Fermi level and the nearest band edge (valence band or conduction band) and in an ideal intrinsic semiconductor material its value is of the order of one-half the energy gap.
As disclosed in U.S. Pat. No. 4,226,898 of Ovshinsky, et al, which patent is assigned to the assignee of the instant invention and the disclosure of which is incorporated by reference, fluorine introduced into the amorphous silicon alloy semiconductor layers operates to substantially reduce the density of the localized defect states in the energy gap thereof and facilitates the addition of other alloying materials, such as germanium. As a result of introducing fluorine into the host matrix of the amorphous semiconductor alloy, the film so produced can have a number of favorable attributes similar to those of crystalline materials. A fluorinated amorphous semiconductor alloy can thereby provide high photoconductivity, increased charge carrier mobility, increased diffusion length of charge carriers, low dark intrinsic electrical conductivity, and, where desired, such alloys can be modified to help shift the Fermi level to provide substantially n- or p-type extrinsic electrical conductivity. Thus, fluorinated amorphous semiconductor alloy materials can act like crystalline materials and be useful in devices, such as, solar cells and current controlling devices including diodes, transistors and the like.
It is now possible to deposit a good n-type conductivity amorphous silicon alloy film. In order to similarly fashion a good p-type conductivity amorphous silicon alloy film, it is necessary to substantially substitutionally and monoatomically incorporate the boron or other p-dopant atoms into the host matrix of the semiconductor alloy along with the semiconductor, fluorine and hydrogen atoms. However, it is difficult to tetrahedrally introduce boron atoms into the semiconductor alloy matrix. Moreover, boron introduced into the matrix through the glow discharge of a diborane precursor gas has a tendency to form chains of higher order boron hydrides which are normally not accepted in said matrix without either breaking and straining existing bonds of the matrix or initiating the undesirable morphology and growth of the depositing films. It is toward the goal of achieving a more perfect tetrahedral introduction of boron and/or lower order (monoatomic) boron species than heretofore possible that the present invention is directed.
Unlike crystalline silicon which is limited to batch processing for the manufacture of solar cells, the aforedescribed, high efficiency amorphous silicon alloys can be deposited in multiple layers over large area substrates to form solar cells in a high volume, continuous processing system. Such continuous processing systems are disclosed in the following U.S. Ser. Nos. 4,400,409, for A Method Of Making P-Doped Silicon Films And Devices Made Therefrom; 4,410,588, for Continuous Amorphous Solar Cell Production System; 4,438,723, for Multiple Chamber Deposition And Isolation System And Method; and pending U.S. patent application Ser. Nos. 244,386, filed Mar. 16, 1981, for Continuous Systems For Depositing Amorphous Semiconductor Material; 359,825, filed Mar. 19, 1982, for Method And Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells; and 460,629 filed Jan. 24, 1983 for Method and Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells. As disclosed in these applications, a substrate may be continuously advanced through a succession of interconnected, environmentally protected deposition chambers, wherein each chamber is dedicated to the deposition of a specific semiconductor material. In making a photovoltaic device, for instance, of p-i-n type configurations, the first chamber is dedicated for depositing a p-type semiconductor alloy, the second chamber is dedicated for depositing an intrinsic amorphous semiconductor alloy, and the third chamber is dedicated for depositing an n-type semiconductor alloy. The layers of semiconductor alloy material thus deposited in the vacuum envelope of the deposition apparatus may be utilized to form photoresponsive devices, such as, but not limited to photovoltaic cells which include one or more p-i-n type cells. Note that as used herein the term "p-i-n type" will refer to any sequence of p and n or p, i, and n semiconductor alloy layers. Additionally, by making multiple passes through the succession of deposition chambers, or by providing an additional array of deposition chambers, multiple stacked cells of various configurations may be obtained and conformally deposited upon an irregularly contoured substrate.
It is important to note that trace amounts of boron are routinely incorporated into the intrinsic semiconductor alloy layer, hence the reason that the intrinsic layer is modified by the term "substantially" in the present application. Applicants' have found it desirable to purposely introduce the trace levels of boron, in the range of approximately ten ppm or less, into at least a portion of the intrinsic semiconductor alloy layer in order to accomplish such functions as, inter alia, increasing the open circuit voltage and increasing the fill factor. The boron, even though incorporated in such low levels, may serve to improve the field profile (thereby improving the collection of charge carriers) and aid in the stabilization of the intrinsic semiconductor alloy layer. Further, the addition of fluorine may also serve to further stabilize said intrinsic semiconductor alloy layer.
Although the reasons for the poor performance of semiconductor alloy layers which were fabricated by incorporating a diborane precursor gas as a source of boron for p-doping the semiconductor alloy material are not fully understood, it is known, as alluded to hereinabove, that even though diborane is a dimer of BH.sub.3, under the influence of the electromagnetic field existing in the plasma region created by the glow discharge deposition process, diborane tends to produce higher boron oligomers and polymers, referred to herein as "non-monoatomic boron species". These higher order boron hydrides are very difficult to decompose in the electromagnetic field developed during the course of the glow discharge deposition process, and consequently tend to be incorporated into the semiconductor alloy material in the form of chains of boron. In this manner, the boron which is incorporated into the host matrix of the semiconductor material tends to alloy with, rather than substitutionally dope, that semiconductor material matrix. It is also believed that the boron atom, which is produced by the glow discharge decomposition of diborane gas, tends to be incorporated in said semiconductor alloy matrix in three-fold coordination. This is because boron, having three valence electrons, has a tendency to form three covalent bonds. Further, the large degree of rotational freedom present in a disordered, semiconductor alloy matrix provides a sufficient degree of freedom in the structural arrangment of atoms for the boron atoms to be incorporated thereinto in trivalent form. Whereas, the boron deposited by such glow discharge decomposition of diborane tends to assume three-fold coordination, as mentioned hereinabove, boron must be four-fold coordinated in order to act as an acceptor, and thereby most effectively dope the semiconductor alloy material. For this reason, effective doping levels of boron derived from diborane can be much less than the actual concentration of boron atoms incorporated into the amorphous semiconductor alloy matrix. As a result of the foregoing discussion, it should be apparent that a high density of strained and broken bonds as well as morphological and growth problems are normally introduced into the matrix of the semiconductor alloy by atoms of boron or molecules of boron species which are incorporated in an oligomeric or a polymeric form, or by atoms of boron or molecules of boron species which are present in three-fold coordinated form; both of which shall be referred to hereinafter as the non-substitutional incorporation of boron and/or boron species into the host matrix of the semiconductor alloy material.
In the manufacture of semiconductor devices, it is obviously desirable to substitutionally incorporate a high concentration of boron into the semiconductor alloy matrix in order to obtain the highest possible operating efficiency of said devices. Accordingly, it is desirable to utilize a precursor dopant material which exhibits as high a degree of substitutional incorporation of the dopant atoms thereof into the semiconductor alloy matrix as possible, so as to provide as low a density of broken and strained matrix bonds as possible. The presence of broken, strained, popped, dangling or other deleterious bonding configurations in the matrix as well as undesirable morphology and growth of the semiconductor film, in even the thin semiconductor alloy layers which are deposited by the aforementioned glow discharge processes, results in the deposition of a semiconductor layer exhibiting high internal stress and/or undesirable morphological growth. Stress strain and the presenced of undersirable morphologies may, in some cases, manifest themselves in the form of cracked, peeling or hazy semiconductor layers as well as degraded electrical performance of devices incorporating those layers; and in situations wherein it is desirable to deposit thicker semiconductor layers, the stress and morphological problems are exacerbated. Moreover, it has been found by Applicants that cloudy and/or hazy semiconductor layers present poor optical properties as well as poor electrical properties. Obviously, if the p-doped layer becomes cloudy, it is absorbing or scattering incident light from the solar spectrum, which light should preferably be transmitted for photoconversion by the intrinsic semiconductor alloy layer. Therefore, this condition cannot be tolerated. Another feature described herein, relates to the substantial elimination of the deposition of such hazy p-doped films of semiconductor alloy material.
It is important to note that while the foregoing Paragraphs deal with commonly recognized scientific beliefs regarding the reasons for poor substitutional doping of amorphous semiconductor alloys using a diborane precursor gas, such beliefs have not as yet attained the status of dogma and represent only one plausible explanation for the inadequate p-doping achieved using a diborane precursor gas. However, regardless of the scientific explanation attributed to the presence of high concentrations of strain and deleterious morphology and growth in p-doped, thin film semiconductor alloys produced by glow-discharge deposition using silane and diborane precursor gases, it is patently clear that (1) a high density of defect states are introduced into the band gap, (2) the band gap of the semiconductor alloy is narrowed, thereby causing the alloy to absorb an undesirably high amount of the incident solar spectrum, (3) semiconductor alloy films so deposited show marked degradation when such films are used as the photogenerative layer of a photovoltaic device, (4) such semiconductor alloy films are very highly stressed which substantially limits the utility of very thick layers, as will be further demonstrated hereinafter.
While the mechanism by which photodegradation of the amorphous semiconductor alloy film is also not fully understood, it is safe to state that said photodegradation produces an increase in the number of defect states in the band gap of the semiconductor alloy material. The term "defects" or "defect states", as generally used by routiners in the field of amorphous semiconductor materials, is a broad term generally including all deviant atomic configurations such as broken bonds, dangling bonds, bent bonds, vacancies, microvoids, etc. In a photovoltaic device, a charge carrier pair (i.e an electron and a hole) is generated in response to the absorbtion of photons from the incident radiation in the photoactive region of the semiconductor alloy material thereof. Under the influence of an internal electric field established by the doped layers of semiconductor alloy material of the photovoltaic device, such as a solar cell, the charge carriers are drawn toward the opposite electrodes of the cell causing the positively charged holes to collect at the negative electrode and the negatively charged electrons to collect at the positive electrode thereof. Under ideal operating conditions, every photogenerated charge carrier would be conducted to its respective collection electrode. However, operating conditions are not ideal and the loss of charge carriers occurs to some degree in all photovoltaic devices. The primary charge carrier collection loss is due to charge carrier recombination wherein an electron and a hole are reunited. Obviously, charge carriers that reunite or recombine are not available for electrode collection and the resultant production of electrical current. Defects or defect states that occur in the photoactive region of the semiconductor material of the photovoltaic device provide recombination centers which facilitate the reunion and recombination of electrons and holes. Therefore, the more defects or defect states that are present in the band gap of the semiconductor alloy material of a device, the higher the rate of charge carrier recombination therein. Accordingly, charge carrier collection efficiency decreases as the rate of charge carrier recombination increases within the photoactive region of the given semiconductor material. The increase in the number of defect states is therefore at least partially responsible for an increase in the rate of charge carrier recombination and a concommitant decrease in photovoltaic cell conversion efficiency.
Such photodegradation upon prolonged light exposure has been experimentally shown to affect many of the electrical and optical properties of hydrogenated fluorinated amorphous silicon alloys, i.e., light induced changes have been observed in dark conductivity, photoconductivity, photoluminescence, spin density, and gap state density. The effects of prolonged light exposure have been found to be metastable so that annealing the semiconductor alloy materials at temperatures above 150.degree. C. for several hours restores the original value of the various properties. It is very noteworthy that there is a difference in the annealing behavior between undoped and very lightly p-doped materials which may be attributed to the structural difference in the two materials. It has been observed that boron doping with diborane (at a level of B.sub.2 H.sub.6 /SiH.sub.4 =50 ppm) weakens the hydrogen bonding configuration. This results in a lower activation energy for both creation and annealing of defects since it is generally believed that the defect creation involves breaking of a weak bond.
In an attempt to eliminate at least some of the aforementioned deleterious effects resultant from diborane doping of amorphous semiconductor alloy materials, scientific investigators have experimented with other boron-containing species, such as boron trifluoride (BF.sub.3), to be used as the boron precursor gas.
In an article published in Journal of Electronic Materials, Volume 12, No. 6 1983 entitled "BF.sub.3 -Doped Amorphous Silicon Thin Films" by A. H. Mahan et al, a method for the radio frequency glow discharge deposition of BF.sub.3 doped amorphous silicon is disclosed. As described therein, boron is incorporated into amorphous silicon alloy films by including boron trifluoride gas in the glow discharge deposition atmosphere created in the deposition chamber. The authors, depositing films from an atmosphere of silane and boron trifluoride, found that at the highest boron levels employed, (i.e., 3.5 percent BF.sub.3 in silane), 1.8 atomic percent of boron was incorporated into the silicon alloy films, said films characterized as having an activation energy of 0.34 electron volts and a band gap substantially similar to that of undoped amorphous silicon alloys. According to the authors, the results represented the largest amount of boron capable of being incorporated into the semiconductor alloy host matrix via the use of BF.sub.3. While samples of amorphous silicon alloys doped with higher boron levels could be prepared utilizing diborane; samples thus prepared exhibited decreased band gaps and a higher density of defect states in the band gap of the semiconductor alloy.
In the discussion section of the Journal article, the authors recognize the advantages of utilizing boron trifluoride as a dopant, especially with regard to the maintainance of a constant band gap in the doped semiconductor alloy material and suggest that boron trifluoride doped silicon may be advantageously utilized to form the p-layer of a p-i-n type photovoltaic device. However, the authors further state that the dopant incorporation ratio decreases steadily as the boron trifluoride concentration is increased and therefore problems are encountered in obtaining high levels of doping utilizing BF.sub.3. Therefore, while Mahan, et al recognize the need for a heavily p-doped silicon semiconductor alloy material having a relatively wide band gap, both (1) their own experiments and (2) their discussion in the paper indicate that BF.sub.3 cannot be utilized to substitutionally incorporate sufficient concentrations of boron for doping the alloy material.
In a paper entitled "BF.sub.3 -Doped Amorphous Silicon Thin Films", G. Devaud, et al of the Solar Energy Research Institute, Golden Colo., also disclose the use of boron trifluoride as a dopant for amorphous silicon alloy films. Although they recognize the utility of BF.sub.3 as a precursor dopant gas which does not alter the band gap of said amorphous alloys, Devaud, et al were unable to incorporate sufficient amounts of boron into their films to fabricate a highly doped p-layer of amorphous semiconductor alloy material.
U.S. Pat. No. 4,409,424 of Devaud, entitled "Compensated Amorphous Silicon Solar Cell" discloses subject matter substantially similar to that of the aforementioned paper. Specifically, the Devaud patent discloses the use of BF.sub.3 only for the compensation mode doping of the intrinsic semiconductor alloy layer of a p-i-n type photovoltaic cell. Diborane, rather than BF.sub.3, was utilized as the precursor gas from which to form the p-doped layer of the p-i-n cell, presumably, because of the inability of BF.sub.3 to sufficiently p-dope the amorphous silicon alloy to a level sufficient to provide for the fabrication of a high quality photovoltaic cell. It will be noted that while the Devaud patent and the Devaud, et al paper disclose the use of deposition atmospheres containing up to 10% BF.sub.3 therein, those atmospheres are still inadequate for the deposition of a p-doped silicon alloy film. Accordingly, the aforementioned references also teach away from the instant invention insofar as they imply that high doping levels cannot be achieved by utilizing BF.sub.3 in combination with silane in a glow discharge process.
It has recently been brought to Applicant's attention that R. V. Kruzelecky, et al, working at the University of Toronto in Toronto, Ontario have done research on the doping of amorphous silicon alloys with boron trifluoride by glow discharge deposition. Kruzelecky, et al determined that boron trifluoride-doped amorphous silicon alloy films exhibit substrate dependent problems of adhesion which they attributed to the use of BF.sub.3. The silicon alloy films of Kruzelecky, et al were prepared by the glow discharge decomposition of mixtures of silane and BF.sub.3. The most heavily doped boron containing film thus obtained had an activation energy of 0.31 eV, thus being substantially similar to the silicon alloy films described above with respect to the Devaud, et al references. Analysis of the films revealed that only approximately 0.2 to 0.5 atomic percent of boron and a similar amount of fluorine were incorporated thereinto. As in all of the silicon alloy films described in the foregoing references, no significant narrowing of the band gap occured with BF.sub.3 doping.
All of the aforementioned references, whether taken singly or collectively, accentuate the desirability of using BF.sub.3 as the precursor gas from which to p-dope amorphous silicon alloys insofar as BF.sub.3 does not substantially narrow the band gap of the semiconductor alloy materials in which it is incorporated, indicating that the boron therefrom is incorporated substitutionally into the host matrix of the semiconductor alloy material. However, the references also proceed to demonstrate the desirability of achieving higher levels of substitutional boron doping than was attained. That is to say, the prior art acknowledges that it would be highly desirable to provide a non-degradable, heavily doped, p-type semiconductor alloy having a relatively wide band gap layer included in a p-i-n type photovoltaic cell; i.e., such a p-layer (1) exhibiting a relatively wide band gap (i.e. approximately 1.7 eV), (2) being relatively unstressed, and (3) producing more desirable chains of boron species when decomposed. It should be noted that while all of the foregoing references speak of the desirability of achieving high doping concentration of boron in the semiconductor alloy material, they (1) acknowledge and demonstrate that they are incapable of p-doping the silicon alloy material to the extent necessary to make said alloy material function as an efficient p-doped layer and (2) incorrectly state that BF.sub.3 increases the stress imparted to the deposited silicon alloy film. Further, relative to the intrinsic semiconductor alloy layer, the references make no mention whatsoever of improved Staebler-Wronski stability.
It should be emphatically noted that the use of the invention disclosed and claimed herein is not limited to photovoltaic devices. P-type semiconductor alloy layers may be incorporated in thin film electronic devices or in relatively thick electrophotographic devices. The employment of the substantially monoatomic boron halides and pseudo-halides, from which to fabricate the multilayered photoreceptor of an electrophotographic device, forms a particularly noteworthy use of the improved semiconductor alloy materials of the instant invention.
As discussed hereinabove, p-type semiconductor alloy material deposited from an atmosphere which includes diborane as a precursor gas demonstrates a marked tendency to form higher order boron hydrides, which higher order species are incorporated into the host matrix of the semiconductor alloy material. The result of incorporating such non-monoatomic, higher order boron species into the matrix of even a thin film semiconductor alloy is the deposition of a semiconductor alloy material which exhibits high stress, undesirable morphologies in those areas of the deposited alloy material which have incorporated those higher order boron species, undesirable growth of the semiconductor alloy material, and an increased propensity to cracking and peeling of that semiconductor alloy material from the subjacent surface upon which it is deposited.
The aforementioned properties, which result from the higher order boron species being incorporated into a thin film matrix, are multiplexed if it is desired to deposit a relatively thick layer of semiconductor alloy material, as is the case with the deposition of a 25 micron thick electrophotographic photoreceptive member. While it is very difficult to measure internal stresses and strains in thin film semiconductor layers, even when such layers are on the order of 25 microns thick, the presence of stress in a semiconductor layer may be inferred. It has been found in the fabrication of electrophotographic media that by varying parameters such as, substrate temperature and reaction gas composition, semiconductor alloys with varying degrees of stress in the matrix thereof may be fabricated. It has further been found that such stressed electrophotographic photoreceptors car characterized by a generally hazy appearance and a low charge storage capacity (i.e. low Vsat.). It has also been noted that such highly stressed films have a tendency to form cracks and to peel away from the substrate. It is generally postulated that the low Vsat is exhibited by stressed layers resultant from charge dissapation ocurring along the boundaries of cracks, and it is also thought that strained regions themselves may provide paths for charge dissipation without actually forming cracks.
Cracks, or strained regions can also act as nucleation centers for the growth of undesirable morphologies, and such regions of undesirable morphology can then perpetuate through the matrix of the semiconductor alloy material to yield large areas of semiconductor material which are alloyed with long chains of higher order boron species. Obviously, it would be desirable to fabricate electrophotographic photoreceptors having relatively thick semiconductor alloy layers therein characterized by a low degree of internal stress desirable growth and morphology.
The principles and advantages of the instant invention will be readily apparent from the drawings, the description of the drawings and the examples which follow.